1. Field of the Invention
This invention relates to the field of disk drives, and in particular to the field of high bandwidth flexures for hard disk drive drives.
2. Description of Related Art
A disk drive generally uses one or more spinning storage disks, sometimes called storage media, to store data. Disks can be rigid, as used in hard drives, or flexible, as used in floppy drives. Disks commonly store data using magnetic methods or optical methods, and can spin at rates exceeding 15,000 revolutions per minute (rpm). Hard disk drives generally employ several rigid disks stacked one on top of another with spaces in between, attached to a common spindle. Floppy disk drives generally employ a single flexible disk in a bonded sleeve.
Over the surface of each disk in a disk drive, commonly on both sides of each disk, a read-write head is suspended in close proximity to the disk surface by a disk drive suspension. A disk drive suspension is sometimes referred to as a disk drive head suspension or simply a suspension. In hard disk drives with multiple disks on a spindle, suspensions operate in the spaces between the disks and on the two outer disk surfaces. A suspension is a cantilever beam-like feature, mounted on a movable actuator arm. The suspension extends to a precise but variable location above a disk. A suspension typically includes a mounting region, a hinge, a load beam, a gimbal, and a flexure.
The combination of a suspension as discussed above, a read-write head, and a base plate which mounts the suspension to an actuator arm is sometimes called a suspension assembly a head suspension assembly (HSA). A base plate is sometimes called a mount plate, mounting plate, or clad arm.
The load beam is a major arm-like part of the suspension which forms part of its structural backbone. An actuator arm supports the load beam at the load beam's proximal end. The term “load beam” refers to a structure which may be unitary or may be composed of separately formed parts which are later affixed to one another.
The gimbal is held by the load beam over the disk. The gimbal retains the read-write head in a precise position near the load beam distal end while allowing the read-write head to pitch and roll slightly. A gimbal can be an integrally formed portion of a load beam, or it can be a separately formed part.
The read-write head, also referred to as a head or a slider, contains the read-write transducer circuitry upon its proximal end. The slider surface facing the disk is designed and reactive-ion etched to define an aerodynamic pattern typically comprising rails that, in conjunction with the spinning disk, generate a positive pressure thereby lifting the slider from the spinning disk surface. The aerodynamic pattern of protrusions on the slider creates the air bearing surface (ABS) which enables the slider to fly at a constant height close to the disk during operation of the disk drive. The resultant boundary layer of air is commonly called an air bearing. The gram force of the load beam pushes the slider toward the disk while the air bearing of the disk pushes away until an equilibrium position is reached. The equilibrium position is designed to be close enough to the disk so that the slider's read-write circuitry can interact with the disk but far enough away to prevent mechanical contact.
A flexure carries the data signals to and from the read-write head. The flexure typically includes a support layer such as stainless steel, an insulating layer such as polyimide, electrical signal conductors which are typically copper, and a cover coating. The flexure may be referred to as a wiring layer or a circuit or one of several branded terms, i.e. Integrated Lead Suspension (ILS), Flex On Suspension (FOS), Integrated Lead Flexure (ILF), Electrical Lead Suspension (ELS), or Additive Circuit Gimbal (ACG). The flexure electrically connects the read-write head, located at the distal end of the suspension, to an actuator flex. The actuator flex, sometimes referred to as a flexible printed circuit (FPC), connects the flexure to a control circuit board mounted on the base of the disk drive. An electrical interconnect formed by the conductors, sometimes called an electrical lead, is supported by the flexure. The electrical interconnect carries electrical signals from the read-write head that are read from the disk across the suspension to the control circuit board. The flexure also carries electrical signals to be written to the disk from the control circuit board across the suspension to the read-write head. The flexure can be integrally formed on a load beam.
Many current hard disk drive flexures use flying lead features (i.e., with no ground plane backing behind bond pads) to make electrical connections between the flexure and the actuator flex. This design has worked successfully for a number of years because the design is relatively easy to bond. However, the impedance of the flying leads has been relatively poorly matched to read-write head impedance because there is no ground plane under the flying leads. Pad sizes and pitch are fixed based on mechanical requirements.
Newer designs being developed in the hard disk drive industry are eliminating flying lead features and requiring hard disk manufacturers to develop single sided bond pad processes to allow the use of thinner copper flexure constructions. As newer designs with lower and tighter impedance requirements are required to achieve higher flexure bandwidths, the impedance discontinuity within the flexure is becoming a larger issue. Waveforms of circuits having fast switching speed are more susceptible to distortion from the signal line impedance than are waveforms of slower switching speed circuits.
To maximize flexure bandwidth, electrical discontinuities within the flexure are minimized by improved impedance matching. A typical method to match impedance in the flexures is to change the trace width and spacing to match the impedance target of the design. Unfortunately, this standard technique generally cannot be used on the bond pad structures, because the pad size and pitch of the pads is controlled by the bonding technology. Typical bond pad size seen in the industry today is 200-300 micron (μm)×600-800 μm with a bond pad pitch in the 500-800 μm range. These sizes are typically driven by hard drive manufacturer requirements and are not adjustable without greatly impacting the bonding process.
U.S. Pat. No. 5,608,591 to Klaassen discloses a method of minimizing impedance discontinuities in a flexure at a bond pad by removing two large sections of back plane behind around the bond pad, as in the reference's FIGS. 20 and 21. Unfortunately, the removal of such large sections of back plane results in a bond pad which is structurally similar to a flying lead. The bond pad is less supported and thus less compatible with single sided bond pad processes. Also, mini-discontinuities of capacitance are formed.
U.S. Pat. No. 6,891,700 to Shiraishi et al. also discloses a method of minimizing impedance discontinuities in a flexure at a bond pad by forming via holes in the back plane behind the bond pads, as in the reference's FIGS. 3, 5, and 7. The via holes allow increased support of the pads but are limited in how much they can alter impedance.